Optical fiber tree and branch network for AM signal distribution

ABSTRACT

The use of a doped optical fiber amplifier enables the transmission of multi-channel AM-VSB television signals. An Erbium doped fiber amplifier is disclosed. The amplifier is also useful for reducing second order distortion products produced in an amplitude modulated subcarrier optical communication system. The amplifier may be used in a tree and branch structure optical fiber television network.

This application is a divisional of copending U.S. patent applicationSer. No. 07/454,772 filed on Dec. 22, 1989, now U.S. Pat. No. 5,210,631.

BACKGROUND OF THE INVENTION

The present invention relates to fiber optic communication systems, andmore particularly to apparatus for transmitting amplitude modulatedvestigial-sideband ("AM-VSB") television signals over an optical fibertelevision system.

Cable television systems currently distribute television program signalsvia coaxial cable, typically arranged in tree and branch networks.Coaxial cable distribution systems require a large number of highbandwidth electrical amplifiers. For example, 40 or so amplifiers may berequired between the cable system headend and an individual subscriber'shome.

The use of a television signal comprising amplitude modulatedvestigial--sideband video subcarriers is preferred in the distributionof cable television signals due to the compatibility of that format withNTSC television standards and the ability to provide an increased numberof channels within a given bandwidth. An undesirable characteristic ofAM-VSB transmission, however, is that it requires a much highercarrier-to-noise ratio (CNR) than other techniques, such as frequencymodulation or digital transmission of video signals. Generally, a CNR ofat least 40 dB is necessary to provide clear reception of AM-VSBtelevision signals.

The replacement of coaxial cable with optical fiber transmission linesin television distribution systems has become a high priority.Production single mode fiber can support virtually unlimited bandwidthand has low attenuation. Accordingly, a fiber optic distribution systemor a fiber-coax cable hybrid would provide substantially increasedperformance at a competitive cost as compared to prior art coaxial cablesystems.

One problem in implementing an optical fiber distribution system,particularly for AM-VSB signals, is that the lasers used to transmit thetelevision signal information inherently introduce distortion, mostnotably as a result of second and third order harmonics produced by thenonlinearities of the laser.

Amplification of optical signals within a fiber optic network has alsobeen a problem in the attempt to distribute AM-VSB television signals.As noted above, amplifiers are required between a cable system headendand a subscriber's home in order to provide signals to the subscriber atan acceptable power level. Semiconductor optical amplifiers of the typetypically used in fiber optic systems produce high levels of distortionproducts that are not compatible with multi-channel AM-VSB videosignals. This is due to the short lifetime of the carrier excited statewithin the semiconductor optical amplifier. The recombination time ofsuch an amplifier operating near 1.3 μm or 1.5 μm is about 1.2nanoseconds, which is short compared to the period of a typical AM-VSBsubcarrier operating in the cable television band of about 55.25 MHz-1GHz.

The dependence of second order distortion on carrier lifetime in asemiconductor optical amplifier is discussed in A.A.M. Saleh, et al.,"Nonlinear Distortion Due to Optical Amplifiers inSubcarrier-Multiplexed Lightwave Communications Systems" ElectronicsLetters, Vol 25, No 1, pp 79-80, 1989. As noted in that article, secondorder nonlinear distortion is a significant problem in proposedlightwave cable television home distribution systems, where the use ofsemiconductor amplifiers to overcome inevitable distribution losses canpotentially degrade system performance appreciably.

The difficulties presented in transmitting multi-channel AM-VSBtelevision signals over fiber optic distribution systems have led othersto propose the use of frequency modulation ("FM") instead of the moredesirable AM-VSB format. See, e.g., R. Olshansky, et al.,"Microwave-Multiplexed Wideband Lightwave Systems Using OpticalAmplifiers for Subscriber Distribution", Electronics Letters, Vol. 24,No. 15, pp. 922-923, 1988; R. Olshansky, et al,. "Subcarrier MultiplexedPassive Optical Network for Low-Cost Video Distribution", presented atOFC 1989; and W. I. Way, et al., "Carrier-to-Noise Ratio Performance ofa Ninety-Channel FM Video Optical System Employing SubcarrierMultiplexing and Two Cascaded Traveling-Wave Laser Amplifiers",presented at OFC 1989. Another proposal has been to convert AM-VSBsignals to a digital format for transmission. Digital transmission ofAM-VSB television signals over an optical communication link isdescribed in U.S. Pat. No. 4,183,054 to Patisaul, et al., entitled"Digital, Frequency-Translated, Plural-Channel, Vestigial SidebandTelevision Communication System".

It would be advantageous to provide an apparatus and method fortransmitting AM-VSB television signals over a fiber optic distributionsystem in analog form. It would be further advantageous to provide ameans for reducing second order distortion in an amplitude modulatedsubcarrier optical communication system. The present invention providessuch advantages.

SUMMARY OF THE INVENTION

In accordance with the present invention, amplitude modulated vestigialsideband signals are transmitted over an optical fiber. Light from alight source, such as a laser, is modulated with a signal having anAM-VSB subcarrier. The modulated light is passed through an opticalamplifier having a long excited state lifetime with respect to theperiod of the subcarrier. Means are provided for coupling the amplifiedmodulated light output from the amplifier to an optical fiber. Theoptical fiber may, in turn, be coupled to a tree and branch structurecable television distribution network.

In one embodiment, the modulating signal comprises a television signalhaving a plurality of television channels, each containing a videosubcarrier having a period of substantially shorter duration than theexcited state lifetime of the amplifier to virtually eliminate theadverse effects of second order distortion components. The amplifier canbe a doped fiber amplifier comprising, for example, an Erbium dopedfiber. The light source is preferably a laser operating at or near awavelength at which the amplifier exhibits a gain peak. A doped fiberamplifier in accordance with the invention can comprise an opticalcoupler having a first input to receive the modulated light, a secondinput for receiving light from a pump laser, and an output for couplinga signal comprising the combined inputs to an amplifying fiber such asan Erbium doped fiber.

The present invention also provides apparatus and a method for reducingsecond order distortion in an amplitude modulated subcarrier opticalcommunication system. In accordance with the invention, a doped fiberoptical amplifier is coupled to an optical communication system. An AMsubcarrier modulated optical beam is passed through the amplifier,whereby second order distortion components in the subcarrier arereduced. The amplifier may comprise, for example, an Erbium doped fiberamplifier, pumped with a laser operating in a known pump band, such as665 nanometers, that is operated at a power level where a portion of theamplifier has a low level of inversion. The signal wavelength is placedin the optical spectrum at a wavelength at which the amplifier exhibitsgain, and may be modulated with an AM-VSB video subcarrier having aperiod of substantially shorter duration than the excited state lifetimeof the amplifier.

An optical fiber tree and branch network is also provided fordistributing AM subcarrier multiplexed information. A first opticalfiber splitter has an input for receiving a modulated light signalcarrying multiplexed AM subcarriers. The splitter has a plurality ofoutputs for distributing the light signal. Means are provided foramplifying the light signal with an optical amplifier having a longexcited state lifetime with respect to the periods of the subcarriers.The network can comprise a plurality of additional optical fibersplitters, each having an input for receiving the light signal from anoutput of a previous splitter and a plurality of outputs fordistributing the light signal. The amplifying means can comprise aplurality of doped fiber amplifiers, each amplifying the light signalinput to a different splitter. The amplifiers can comprise Erbium dopedfiber amplifiers. The light signal may be provided by a laser modulatedwith a cable television signal containing a plurality (e.g., 30 or more)channels of AM subcarrier video information.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the apparatus of the presentinvention;

FIG. 2 is an energy level diagram of an Erbium doped fiber amplifier;

FIG. 3 is a graph showing the spontaneous emission spectra of an Erbiumfiber amplifier that has been used in the testing of the presentinvention;

FIG. 4 is a graph showing the stimulated emission spectra of a 1536nanometer signal source at the output of the Erbium doped fiberamplifier;

FIG. 5 is an input RF spectrum illustrating second, fourth and fifthorder distortion products;

FIG. 6 is the RF spectrum of FIG. 5 with reduced second ordercomponents, produced at the output of an Erbium fiber optical amplifierin accordance with the present invention; and

FIG. 7 is a block diagram illustrating the use of doped fiber amplifiersin a tree and branch cable television distribution network.

DETAILED DESCRIPTION OF THE INVENTION

The apparatus and method of the present invention enable thetransmission of a multi-channel AM-VSB television signal over an opticalfiber. Second order distortion components produced in the videosubcarriers by a signal laser are reduced.

A modulated light source 10 can comprise a semiconductor laser 12 orother laser coupled to pass a coherent light beam at a specifiedwavelength through an optical isolator 14. Isolator 14 prevents opticalreflections. This allows the laser to operate in a stable manner. Anexternal modulator16, such as a Mach Zehnder modulator receives thelight from laser 12 and modulates the beam with an RF signal applied atterminal 18. The RF signalmay comprise, for example, a multi-channelAM-VSB television signal. Another optical isolator 20 is provided at theoutput of modulator 18. Isolator 20 buffers the modulator from a fibercoupler 22 in a conventional manner. Those skilled in the art willappreciate that other types of modulated light sources can besubstituted for the laser and external modulator light source 10. Forexample, a DFB laser or external cavity laser can be directly modulatedwith an RF signal as well known in the art.

The output from modulated light source 10 is coupled to one input of afiber coupler 22. Another input of fiber coupler 22 is driven by a pumplaser 24, which operates in a known pump band for a doped fiber 28 thatforms an optical amplifier in combination with pump laser 24 and fibercoupler 22. Fiber 28 is coupled to the output of fiber coupler 22 via aconventional fiber connector 26. Doped fiber 28 is preferably asingle-mode fiber, and in the preferred embodiment is a silica-basedErbium (Er³⁺) doped fiber. Other rare earths and other host glasses arepossible. For example, Nd in ZBLN glass can be used. The output oftheamplifier is coupled to a conventional optical fiber 32 using aconnector 30 well known in the art. The signal distributed over opticalfiber 32 is received at fiber optic receiver 34 (e.g., at a CATVsubscriber location) and the information contained therein is output atterminal 36 for its intended end use.

The specifications for the various components illustrated in FIG. 1 aredependent on the requirements of the intended application of the system.In a CATV application, the doped fiber amplifier must be capable ofsupporting AM subcarrier video information. In order to accomplish this,the excited state lifetime of the amplifier must be long with respect tothe period of the subcarrier itself. An Erbium doped fiber has anexcited state lifetime of approximately 15 milliseconds, and has beenfound to operate well in connection with AM subcarrier transmission ofmultiple channel video. Tests in connection with the development of thepresent invention showed that for a twenty-tone (channel) test, thecomposite second order products ("CSO") and composite triple beatproducts ("CTB") were lower than 55 dBc. Other tests indicate thatintermodulation distortion does not increase as the amplifier begins andoperates under conditions of gain saturation. Thus, it appears that theactual saturationpower of the fiber amplifier is not too important solong as adequate powerlevel and gain are supported. For AM-VSB videotransmission an amplifier gain of 14-17 dB and power levels of +10 dBmto +16 dBm should prove satisfactory for use in a CATV distributionnetwork. It is preferable thatsuch a network maintain intermodulationdistortion at less than 60-70 dBc for both CSO and CTB products. It isalso preferable that the carrier-to-noise ratio ("CNR") be greater than60 dB for each channel, andthat at least 40 subcarrier channels besupported.

The present invention resulted in part from applicants' discovery thatfiber optical amplifiers appear to demonstrate coherent subcarrierintermodulation distortion, and the recognition that this phenomenon isuseful in reducing the second order products produced in amplitudemodulated subcarrier optical communication systems. As noted, thetransmission of AM subcarrier multiplexed video information placesstringent requirements on the optical source and on any opticalamplifier in the transmission link. The dominant distortion productsproduced by directly modulated semiconductor lasers are second order.Typical state ofthe art semiconductor lasers have composite second orderproducts that are too large for 40 channel AM subcarrier videotransmission. In accordance with the present invention, the second orderproducts may be reduced by passing the AM subcarrier modulated opticalbeam through a doped fiber optical amplifier. This has been verifiedthrough the testing of an Erbium(Er³⁺) fiber optical amplifier in thestructure illustrated in FIG. 1.It is noted that other structures mayalso be used. For example, the beam from pump laser 24 can becounter-propagated through the doped fiber with respect to the beam frommodulated light source 10.

In testing the structure illustrated in FIG. 1, pump laser 24 was a dyelaser operating at 665 nm. Pump laser 24 was a Coherent Laser model 702modified to produce continuous wave light at 665 nm. The pump power wasabout 100 mW. Doped fiber 28 was a step index single mode silica-basedfiber having a length of 2.5 meters, a cutoff wavelength of about 1.25μm, an Er³⁺ dopant concentration of about 447 ppm, and anumericalaperture ("NA") of 0.16. Coupler 22 was a wavelength divisionmultiplexing ("WDM") fused-fiber coupler. Such couplers are availablefrom Corning Glass Works under the designation "Multiple Index Coupler".Each of the system connectors was a Radiall series VFO-DF connector(part number F716 002 000) having a reflected power below 55 dB. An RFspectrum analyzer wascoupled to the output of fiber optic receiver 34 atterminal 36. Fiber 32 was a conventional telephone communication fiber,manufactured by Corning Glass Works and designated SMF-21 fiber.

An energy level diagram 40 for an Erbium fiber amplifier is illustratedin FIG. 2. At a pump wavelength of 665 nm, the energy of Erbium ionsincreased from a level E₁ shown at 42 to an energy level E₃ shown at 44.Relaxation then occurred, as indicated at 50, to an excited state E₂shown at 46. Spontaneous emission occurred as indicated at 52, resultingin an excited state lifetime of approximately 15 milliseconds. Anadditional energy state E₄, as shown at 48, was alsoreached resulting ina relaxation 54 and a green fluorescence 56 outside ofthe operatingrange of the present system. Those skilled in the art will appreciatethat the pump laser may be operated at different wavelengths, such as980 nm or 1480 nm instead of the 665 nm wavelength used in the presentexample.

The spontaneous emission spectra for the amplifier is shown in FIG. 3.Curve 60 illustrates the emission from the amplifier while being pumpedbypump laser 24, but with no input from modulated light source 10. Asshown, the spectra includes two gain peaks. Gain peak 62 occurred atapproximately 1536 nm and gain peak 64 occurred at approximately 1552nm. In using the fiber amplifier in a communication system, it ispreferable to operate the amplifier at or near a gain peak to achievethe desired amplification. When the amplifier is used in thedistribution of a multi-channel AM-VSB television signal, high gain isnot as important as in other potential applications. It may bepreferable to operate the amplifier at gain peak 64 (e.g., around 1552nm) which exhibits a higher saturation power, allowing the use of signallasers with higher power. It is noted that for particular applications,the spontaneous emission of thefiber amplifier, and therefore thewavelengths at which the gain peaks occur, can be adjusted by addingco-dopants (e.g., alumina or germanium) to the doped fiber.

Testing of the amplifier was conducted at a wavelength of 1536 nm,corresponding to gain peak 62 of FIG. 3. FIG. 4 illustrates the outputof the optical amplifier showing an amplified signal from modulatedlight source 10 at 1536 nm. Light source 10 was operated to provide asignal power of 8.91 μw to the amplifier. Curve 66 includes theamplified signal 68 as well as the 1552 nm gain peak 64 resulting fromthe spontaneous emission of the fiber amplifier.

FIG. 5 illustrates the RF spectrum generated at the output of lightsource 10 when modulated with an RF input signal having two fundamentalfrequencies of 121.25 MHz and 253.25 MHz, respectively. The spectrum wasobtained by coupling the output of optical isolator 20 directly to afiberoptic receiver. The output of the fiber optic receiver was, inturn, coupled to an RF spectrum analyzer. As shown in FIG. 5, the RFspectrum contained second, fourth and fifth order harmonics. Ofparticular interestare the second order harmonics 72, 74, and 76.

FIG. 6 illustrates the RF spectrum produced at the output of the opticalfiber amplifier. This spectrum was measured at terminal 36 shown inFIG. 1. As indicated, each of the second order distortion products 72',74', and 76' are reduced by 12 dB to 20 dB.

Although it is not fully understood why the doped fiber amplifier of thepresent invention reduces the second order components of an input AM-VSBmulti-channel television signal, the magnitude of the reduction issignificant. One possible explanation is that the amplifiers demonstratecoherent subcarrier intermodulation distortion, wherein the second orderdistortion products are 180° out of phase with the second order productspresent in the input signal. It may also be that the reduction insecondorder distortion results from low inversion of portions of thedopedfiber amplifier. If this proves to be the case, pump laser 24should be operated at a power level insufficient to fully invert theamplifier.

In operation with a multi-channel AM-VSB television signal distributionsystem, it is anticipated that a pump laser having a wavelength ofeither 980 nm or 1480 nm will be used. A possible advantage of a 1480pump laser wavelength is that single mode fibers are currently availableat this wavelength. However, the noise figure (NF) at this wavelength ishigher than that obtainable at a wavelength of 980 nm. A 980 nmwavelength is also advantageous since it is further from the anticipatedoperating frequency of the signal input laser, at about 1552 nm. It maytherefore bepreferable to develop fiber that is single moded at 980 nmto ensure efficient fiber pumping. The development of a 980 nm laserwith sufficientreliability would also be required.

FIG. 7 illustrates an optical fiber tree and branch network fordistributing AM subcarrier multiplexed information. A laser 80 ismodulated with a signal input at terminal 82 to produce a modulatedlight signal. The modulating signal input to terminal 82 can comprise,for example, an RF cable television signal containing 30 or morechannels of AM subcarrier video information. A frequency spectrum forsuch a signal isillustrated at 84.

The modulated light signal is passed through an optical isolator 86which is coupled to an optical fiber 88. The light signal from fiber 88is connected to a fiber optical amplifier 90, such as the Erbium dopedfiber amplifier discussed in connection with FIGS. 1-6 above. The outputof fiber optical amplifier 90 is coupled via optical fiber 92 to a firstoptical fiber splitter 94. Splitter 94 can split the incoming signalinto any number of outputs. For purposes of illustration, a 1×16splitteris shown. Splitter 94 will result in a signal loss, which may,for example,be on the order of 14 dB. If fiber optical amplifier 90 isselected to provide a +13 dBm output level, the light signal at theoutputs of splitter 94 will be at a nominal level of -1 dBm. Another 3dB of link loss is allowed in the fiber optic coupling between theoutput of splitter94 and the input of another fiber optical amplifier96. As shown in FIG. 7,each of the outputs of splitter 94 is coupled toa fiber optical amplifier 96, which in turn may be coupled to anothersplitter in the distribution network. A splitter 98 is illustrated withrespect to one of the outputs of splitter 94. By providing fiber opticalamplifier 96 with a gain of 17 dBm, the accumulated 4 dB loss at theinput of amplifier 96 is compensatedfor, and the signal strength at theinput of splitter 98 is again +13 dBm with respect to the originalsignal strength. Gain, noise and power characteristics of the fiber canbe adjusted to be mutually compatible by adjusting various parametersincluding pump power, rare earth dopant concentration, fiber length andrare earth dopant profile.

A fiber optical amplifier 100 at the output of splitter 98 is designedto compensate for the accumulated loss resulting from splitter 98 andthe coupling between splitter 98 and amplifier 100, to provide a +13 dBmsignal strength at the input to yet another splitter 102 present in thedistribution network. Those skilled in the art will appreciate thatvarious splitters are used in a tree and branch distribution system ofthetype illustrated in FIG. 7. A fiber optical amplifier is provided ateach output of each splitter to return the signal strength to a desiredlevel. In the example illustrated in FIG. 7, signal splitter 102 has aplurality of outputs 104 that are fiber optic distribution end points,coupled via abroadband fiber optic receiver, to existing coaxialdistribution equipment or a subscriber CATV converter.

One advantage of optical distribution as shown in FIG. 7 is that highbandwidth electrical amplifiers are not required between the cabletelevision headend and the fiber optic distribution end point where thesignal is converted back to the electrical domain. Elimination of thehighbandwidth electrical components leads to a more reliabledistribution network. Ultimately, a totally optical communicationnetwork is envisionedwherein multichannel AM-VSB television signals aredistributed from the headend to each subscriber location solely onoptical fiber.

It will now be appreciated that the present invention enables thetransmission of an AM-VSB signal over an optical fiber. The use of adopedfiber optical amplifier having a long excited state lifetime withrespect to the period of the AM-VSB signal overcomes the priorlimitation of semiconductor amplifiers which introduce detrimentalsecond order nonlinear intermodulation distortions. A further benefitderived from the use of the doped fiber optical amplifier is that it hasbeen found to reduce second order distortion already present in an inputsignal. The amplifier may therefore be used in a fiber opticcommunication system for second order distortion reduction.

Although the invention has been described in connection with a preferredembodiment thereof, those skilled in the art will appreciate thatnumerousmodifications and adaptations may be made thereto withoutdeparting from the spirit and scope of the invention as set forth in thefollowing claims.

We claim:
 1. An optical fiber tree and branch network for distributingAM subcarrier multiplexed information comprising:a first optical fibersplitter; an input on said splitter for receiving a modulated lightsignal carrying multiplexed AM subcarriers; a plurality of outputs onsaid splitter for distributing said modulated light signal; and meansfor amplifying the modulated light signal with an optical amplifier. 2.A network in accordance with claim 1 comprising:a plurality ofadditional optical fiber splitters, each having an input for receivingsaid modulated light signal from an output of a previous splitter and aplurality of outputs for distributing the modulated light signal,wherein said amplifying means amplifies the modulated light signalbefore it is input to each additional splitter.
 3. A network inaccordance with claim 2 wherein said amplifying means comprises aplurality of doped fiber amplifiers, each amplifying the modulated lightsignal input to a different splitter.
 4. A network in accordance withclaim 3 wherein said amplifiers are Erbium doped fiber amplifiers.
 5. Anetwork in accordance with claim 1 wherein said modulated light signalis provided by one or more lasers modulated with a cable televisionsignal containing at least 30 channels of AM subcarrier videoinformation.
 6. A network in accordance with claim 1 wherein saidoptical amplifier has a long excited state lifetime with respect to theperiods of said subcarriers.
 7. A network in accordance with claim 1wherein said optical amplifier is a doped optical fiber amplifier.
 8. Anetwork in accordance with claim 7 wherein said optical fiber amplifieris doped with a rare-earth element.
 9. A network in accordance withclaim 8 wherein said rare-earth element is erbium.